readelf: Only lookup files and show errors in attr_callback when not silent.

readelf: Only lookup files and show errors in attr_callback when not silent.

We recently added a couple more sanity checks and show an error when
attributes that represent files cannot be resolved. We should only do
this when the attr_callback should print the attribute information.
When not (for example when not showing the .debug_info section, but
only collecting information about the CUs containing pointers into
other sections) we don't need to lookup the file names, nor print
the errors/warnings.

Signed-off-by: Mark Wielaard <mark@klomp.org>